Unfortunately human activity has led to a widespread distribution of pollutants in the environment. Persistent organic pollutants (POPs) are accumulated through the food chain, making predatory animals and humans particularly vulnerable to their harmful effects. Of special concern are fat-soluble persistent environmental pollutants, which end up in the fat tissue of organisms like fish and sea mammals. Unfortunately, this type of chemicals frequently can be found in body oils and liver oils from fish, as well as in oil from marine mammals like seals and whales. Farmed fish, like salmon and trout, are exposed to pollutants through fish oil used in their feed. If consumed by humans, the pollutants may accumulate in the human organism. Because of the extremely slow degradation rate, persistent environmental pollutants will tend to increase as the individual gets older.
POP exposure can cause death and illnesses including disruption of the endocrine, reproductive and immune systems, neurobehavioral disorders; and cancers. In pregnant and lactating women, POP can be transferred to the fetus or infant baby through the mother or the mother's milk, potentially harming the developing infant.
The level of pollutants in nature raises concerns, both for nature and wildlife itself, but also for human consumption of oils from such sources. Therefore there is a need for methods to reduce the concentration of pollutants in oils for human or animal consumption.
Animal oils, including fish oils, contain cholesterol. Frequently it is necessary or desirable to remove cholesterol before edible oils are used for food, health supplements or pharmaceuticals.
WO 2004/007654 describes a process for decreasing environmental pollutants in an oil or fat by adding a volatile working fluid before subjecting the oil to a stripping process, e.g. short path evaporation, molecular distillation or a similar process. WO 2004/007655 describes a related type of process, where e.g. fatty acid ethyl or methyl esters are used as a working fluid, for the reduction of cholesterol.
According to these documents, the addition of an external volatile working fluid to a triglyceride oil before molecular distillation gives a more efficient process, resulting in higher removal rates of pollutants or cholesterol. Addition of a volatile working fluid also allows for the use of lower temperatures and/or increased capacity of the production equipment. Reduced process temperatures and holding times (resulting from increased flow rates) are important to prevent degradation of the double bonds of polyunsaturated fats, like EPA and DHA in marine oils.
WO 2004/007654 also describes the use of free fatty acids naturally contained in marine oils as an internal working fluid in a stripping process. Thereby, the addition of an external working fluid can be avoided.
However, using free fatty acids contained in the untreated crude oil as an internal working fluid also has disadvantages. Crude oil products normally contain small amounts of hydrophilic components such as water-soluble proteins, peptides etc. Such proteinaceous or other components might cause problems when present in a stripping processing step since they tend to burn into the heating surfaces of the equipment used for short path evaporation or molecular distillation, where temperatures frequently can be about 200° C. or higher.
In commercial full scale production huge volumes of crude oils will normally be processed. Even if concentrations of proteins, peptides and other water-soluble components in the crude oil are small, processed oil volumes normally will be high, and build-up of scaling, i.e. deposition of particulate matter, from such hydrophilic components on the evaporator heating surfaces might after a relatively short time create technical or quality problems.
Examples of potential problems are reduced heat transfer from the heating medium to the oil film, impurities of loosened scaling in the stripped oil, restrictions in the flow lines etc. Such problems might be solved by increasing the frequency of cleaning the equipment. However cleaning of evaporators and other equipment that has been used for stripping of marine oils is not a trivial matter. If using untreated crude oil, containing free fatty acids as an internal working fluid, the equipment might for instance have to be cleaned at least on a weekly or bi-weekly basis instead of for instance every three months. The necessary cleaning frequency will vary with use, requirements, incoming oil quality and throughput in the equipment.
To fully remove heavy scaling from this type of equipment in-line cleaning with a caustic cleaning agent will normally not be sufficient, but dismantling the system with removal of the internal wiper unit might be necessary. For industrial scale equipment removal of a vertically mounted, several meters long, stainless steel wiper unit for cleaning is a time consuming process. After cleaning it is important to take care that all connections are without even small leaks, and water must be completely removed from the system to allow generating the deep vacuum needed for the stripping process. For these reasons cleaning of such systems is very time-consuming and costly. Reduced frequency of cleaning therefore is very important to reduce down-time, and can be vital for productivity and process economics.
In up to date commercial processes, the crude oil is deacidified by alkali refining prior to the stripping process thereby removing the free fatty acids present in the crude oil. In addition, alkali refining will also wash out hydrophilic components, such as proteinaceous matter, thereby reducing build-up of scaling in the equipment used for stripping. However, since the free fatty acids are removed before stripping, an external working fluid has to be added for the stripping process.